filter
I would use a canister filter such as the smallest eheim ecco model (2231 ?) or the small classic model 2213. I would just put the return line under the water surface. I like the canisters since they take up less room in the tank.
If you use an internal filter or a canister filter, you should be able to hook the output up to a DIY co2 reactor which is a big plus.
Then again, if the fish load is light and the plants are healthy, you may eventually be able to run with little to no filtration.